This application claims the priority benefit of Taiwan application serial no. 91107694, filed on Apr. 16, 2002.
1. Field of Invention
The present invention relates generally to semiconductors and more specifically to a method for fabricating contact plug.
2. Description of Related Art
Integrated circuits are manufactured as assemblies of the various devices, such as transistors that make up a chip and many chips are included on a single wafer. In the process of manufacturing integrated circuits, after the individual devices, such as the transistors, have been fabricated in the silicon substrate, they must be connected together to perform the desired circuit functions. This connection process is generally called xe2x80x9cmetallizationxe2x80x9d, and is performed using a number of different photolithographic and deposition techniques.
The contact plugs are formed to make a solid electrical connection between the underlying device and the overlying interconnection conductive line The fabrication of contact typically involves forming an opening in a dielectric layer and the opening is filled or xe2x80x9cpluggedxe2x80x9d with a metallic layer, such as aluminum or tungsten. However, aluminum or tungsten ions from the contact can migrate into a silicon substrate through a doped region, causing a short to the substrate. To minimize this shorting, some processing techniques deposit a barrier layer before depositing the aluminum or tungsten. One type of barrier material is TiN. While TiN has a good barrier ability, but it needs to be thick enough to effectively function as a barrier layer. Furthermore, as integrated circuit devices are defined more finely, diameter of the contact shrinks and becomes more critical. Thus, thick TiN barrier metal layer is less desirable in high integrated circuits. It has been found that nitride in TiN improves the barrier function, in other words, as the nitride content in TiN increases, the barrier effect also increases. One approach is to implant nitrogen into TiN in order to increase the barrier effect and reduce the thickness of the TiN barrier metal layer in order to meet high integrated structure requirement. One method for implanting the nitrogen into TiN is to perform a nitrogen plasma treatment in an atmosphere of hydrogen gas. Now it is possible to form thinner TiN barrier metal layers meeting the high integrated circuit requirement.
Another commonly used barrier metal layer is formed from metal organic CVD titanium nitride (MOCVD-TiN). Inherently, the MOCVD-TiN material contains impurities such as carbon and oxides, therefore the resistance of MOCVD-TiN material is high. In order to reduce the resistance, one method is to remove these impurities by treating the said barrier layer with a plasma gas containing an atmosphere of nitrogen. However, following the plasma gas treatment, the thickness of the MOCVD-TiN is substantially reduced, consequently, the treated MOCVD-TiN layer so formed has comparatively lower resistance but however the thickness is not adequate to function as a barrier layer effectively.
The effectiveness of the contact is limited by the contact resistance between the barrier metal layer and the doped regions in the substrate. This contact resistance is greater for positively doped regions than for negatively doped regions. Contact resistance is of particular concern in CMOS (complementary metal-oxide-silicon) technology, which includes barrier metal contacts with positively doped regions as well as with negatively doped region. One approach to reduce the contact resistance is to deposit a conformal refractory metal layer into the opening and then annealing the refractory metal layer by performing a thermal process in order to effect a reaction between the metal and the silicon atoms to form metal-silicide. Since metal-silicide has a low resistance, consequently the contact resistance can be reduced. However, one problem with the above conventional contact plug scheme is that the resulting barrier layer following the thermal process offers poor adhesion between the dielectric layer and the tungsten layer.
Applicants have found out that during the thermal process, the oxygen from the ambient reacts with the barrier metal layer to form an oxide film on the surface of the contact barrier metal layer. One problem with the oxide film so formed is, they have a zeta potential value approximately same as the tungsten layer which is commonly used for filling into the contact opening. Because of the similar zeta potential value of the barrier metal layer and the tungsten layer, they tend to repel from each other.
Therefore, the oxide film formed on the barrier metal layer prevents the tungsten layer from adhering onto the surface of the barrier metal layer. Consequently voids are formed within the tungsten layer leading to electromigration failure. Because the oxide film has a poor adhesion property due to zeta potential, adhesion between the dielectric layer and the tungsten layer in the contact is poor. As difference in thermal coefficient of expansion between conductive layer such as tungsten and the dielectric layer is large, therefore during the subsequent thermal process, the thermal stress due to thermal expansion is large. Consequently, the conventional barrier metal layer is not strong enough to resist the thermal expansion and are fractured. Consequently, because the barrier metal layer structure is damaged, it promotes the diffusion of ions or atoms from the conductive layer such as aluminum or tungsten atoms into the substrate, causing shorting of device. Because the lattice structure of the conductive layer has been damaged, it promotes voids formation due to electromigration causing total failure of device.
Therefore in the foregoing problems as described above, the present invention provides a solution to solve the above problems.
The present invention provides a method for fabricating contact plug so as to eliminate the electro-migration of conductive material in the contact.
The present invention provides an improved method for forming contact plug to reduce the contact resistance. Therefore the RC delay time can be reduced, thus the operating speed of the device can be substantially increased.
The present invention provides a method of contact barrier metal layer to improve the adhesion ability between the dielectric layer and the conductive layer so that cracking or fracturing of barrier metal layer can be prevented. Thus increasing the reliability of the semiconductor device.
The present invention provides a method of contact barrier metal layer to improve the gap-fill ability of the conductive material and to increase adhesion ability between the dielectric layer and the conductive layer so that generation of voids can be prevented. Thus device failure due to electromigration can be effectively prevented thereby increasing the reliability of the semiconductor device.
The present invention provides a method for forming a contact metal barrier layer so that the adhesion ability between the dielectric layer and the conductive layer in a contact can be increased. Thus eliminating the defects due to electromigration, thereby increasing the reliability of the semiconductor device.
According to one of the preferred embodiment, the present invention provides an improved method for fabricating contact plug. A semiconductor substrate having a conductive region is provided, a dielectric layer is formed over the entire substrate, the dielectric layer is etched to form a contact opening, wherein the conductive region is exposed within the contact opening. A pre-clean process is carried out to remove the residues which would otherwise increase the contact resistance. The contact opening is coated with a first refractory metal layer. Next, a second refractory metal nitride layer is deposited over the first refractory metal layer, then a first plasma treatment is carried out to transform the second refractory barrier metal layer into a nitrided barrier layer. The first plasma treatment preferably comprises a plasma gas including nitrogen and hydrogen gas. A thermal-process is carried out in order to effect a reaction between the silicon atoms of the conductive region and the first refractory metal layer to form a metal-silicide in order to reduce the contact resistance. Then a third refractory metal nitride layer is deposited on the first metal nitride barrier layer and similarly treated with a plasma gas including nitrogen and hydrogen gas to remove the impurities from the third refractory metal nitride layer and to transform the third refractory metal nitride layer into a second nitrided barrier layer and then a conductive layer is deposited filling the contact opening.
It is to be understood with the approach of the present invention that by performing a thermal process is to effect a reaction between the silicon in the conductive region and the first refractory metal layer is to form a metal-silicide layer. Because the metal-silicide has a low resistance, therefore the contact resistance can be substantially reduced. Thus the operating speed of the device can be substantially increased.
Further, it is to be understood with the approach of the present invention that by performing a thermal process and then forming a second metal nitride barrier layer is to eliminate the formation of oxide film on the surface of the second metal nitride barrier layer so that the repulsion between the second metal nitride barrier layer and the conductive layer can be effectively eliminated in order to facilitate good gap fill of the conductive layer and to promote adhesion between the dielectric layer and the conductive layer. Because the zeta potential values of the second metal nitride barrier layer and the conductive layer are largely different, there will be no repulsion between them, consequently no voids are generated, thus the electromigration failure can be effectively prevented. Because the adhesion between the dielectric layer and the conductive layer is increased, cracking of the first refractory metal layer, the first and second metal nitride barrier layers can also be effectively prevented. Because the first refractory metal layer, the first and the second metal nitride barrier layers are not cracked or fractured, therefore the first and second metal nitride barrier layers can effectively prevent the diffusion of metal ions or atoms into the dielectric layer, thus shorting of devices can be prevented. Since the lattice structure of the conductive layer in the contact is not damaged and the adjacent first refractory metal layer, the first and the second metal nitride barrier layers are undamaged, generation of voids due to electromigration can be prevented, thus defects due to electromigration can be eliminated. Therefore the reliability of the device can be substantially increased.
The above and additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description when taken in conjunction with the accompanying drawings.